Remove instances of generic "Dear user" message

[johnbillion]

​Several of the email notifications which WordPress sends out include the greeting "Dear user", which is commonly associated with spam or phishing emails rather than legitimate emails.

Let's see if we can replace these instances with the user's display name, or remove the greeting altogether where we don't have any user information.

Good first bug for someone who wants to take it.

[Ipstenu]

With Username

[Ipstenu]

Without username

[Ipstenu]

31217-ms-delete-site.diff and 31217-ms-delete-site-2.diff are basically one with and one without the username. The rest already had the logic in to pull in USERNAME or ###USERNAME### so I reused that and did a "Howdy USERNAME" to match the toolbar.

[SergeyBiryukov]

Replying to Ipstenu:

31217-ms-delete-site.diff and 31217-ms-delete-site-2.diff are basically one with and one without the username.

31217-ms-delete-site.diff​ seems like the way to go.

Could you create a single patch with all the changes for easier review?

[Ipstenu]

Full patch, all changes

[Ipstenu]

Replying to SergeyBiryukov:

Could you create a single patch with all the changes for easier review?

Done.

[SergeyBiryukov]

In 31403:

Replace generic "Dear user" greeting in email notifications with a more personalized one.

props Ipstenu.
fixes #31217.

[SergeyBiryukov]

In 31404:

Remove redundant string with a typo introduced in [25481].

see #31217.

[ocean90]

In 33569:

Multisite mails: Initialize $current_user before accessing $current_user->user_login.

props Toro_Unit.
see #31217.
fixes #33254.